1. Field of the Invention
The present invention relates to an electrolyzing electrode for use in an industrial and civilian-use electrolysis process and a process for the production thereof. More specifically, it relates to an electrolyzing electrode which is for electrolyzing a metal electrolytic solution for carrying out a plating, which is used as an anode for a reaction to generate oxygen on the anode and is excellent in durability in the reaction, and which has excellent durability even when it is placed in a poor potential region, and a process for the production thereof.
2. Prior Art of the Invention
Conventionally, a metal electrode formed by providing metal titanium as an electrically conductive substrate and forming a coating of a metal coming under the group of platinum or its oxide thereon is used in the various fields of electrolysis industry. For example, an electrode formed by coating a titanium substrate with oxides of ruthenium and titanium or oxides of ruthenium and tin by a pyrolysis method is known as an anode for generating chlorine by the electrolysis of sodium chloride (Japanese Patent Publications Nos. 46-21884, 48-3954 and 50-11330 and JP-A-52-63176).
The above electrode is suitable for the electrolysis of an aqueous solution containing a high concentration of sodium chloride such as the electrolysis of sodium chloride. In the electrolysis of an aqueous solution containing a low concentration of sodium chloride and the hydrolysis of sea water, however, the above electrode has no sufficient durability, and the efficiency of chlorine generation is not fully satisfactory, either.
Further, as an electrode for the electrolysis of an aqueous solution of an alkali metal halide such as sodium chloride, JP-A-55-152143 and JP-A-56-150148 disclose an electrode formed of an amorphous alloy as an electrode material. However, the amorphous alloy requires a large-scale apparatus for producing the same.
In addition to the electrolysis which involves the generation of chlorine such as the above electrolysis of sodium chloride, the electrolysis industry in various fields uses electrolysis processes which involve the generation of oxygen, such as the recovery of an acid, an alkali or a salt, the collection and purification of a metal such as copper or zinc, plating, the production of a foil of a metal such as copper, the treatment of a metal surface, the prevention of corrosion of a cathode and the disposal of a waste. In the above electrolysis which involves the generation of oxygen, as an insoluble electrode, there are used iridium-oxide-based electrodes or platinum-plated titanium electrodes, such as an electrode formed by coating a titanium substrate with iridium oxide and platinum, an iridium oxide-tin oxide electrode and an iridium oxide-tantalum oxide electrode.
When an electrode formed by coating a titanium substrate is used as an anode to carry out the electrolysis which involves the generation of oxygen, generally, the anode is passivated due to the formation of a titanium oxide layer between the substrate and a coating layer, and the titanium substrate is corroded, so that the anode potential gradually increases, which results in the end of the lifetime of the anode. Further, the coating layer may peel off. For inhibiting the formation of titanium oxide on the titanium substrate and the corrosion of the titanium substrate to prevent the passivation of the anode, there have been made various proposals in selecting proper coating layers or forming proper undercoating layers.
For example, JP-A-5-287572 proposes an electrode for the generation of oxygen, which electrode comprises an electrically conductive substrate, an iridium oxide/tantalum oxide undercoating layer which contains, as metals, 8.4 to 14 mol % of iridium and 86 to 91.6 mol % of tantalum and is formed on the electrically conductive substrate, and an iridium oxide/tantalum oxide overcoating layer which contains, as metals, 80 to 99.9 mol % of iridium and 0.1 to 20 mol % of tantalum and is formed on the above undercoating layer.
Further, JP-A-5-171483 proposes an anode for the generation of oxygen, which anode comprises an electrically conductive substrate, an intermediate layer which is composed of metal tantalum and/or tantalum alloy as main component(s) and is formed on the electrically conductive substrate by plasma spray coating with metal tantalum and/or a tantalum alloy powder and an electrode activation layer which contains at least 20% by weight of iridium oxide and a balance of tantalum oxide and is formed on the above intermediate layer.
Japanese Patent Publication No. 2574699 proposes an electrode for the generation of oxygen, which electrode comprises an electrically conductive substrate, an intermediate layer which is composed of crystalline metal tantalum and is formed on the electrically conductive substrate by a sputtering method and an electrode activation layer which contains a metal coming under the group of platinum or its oxide (iridium oxide, etc.) and is formed on the intermediate layer.
Meanwhile, in metal plating, anodes consist of a pair of flat plates parallel with each other, and a board to be plated is carried therebetween. When both the surfaces of the board are plated, the two electrodes are used as positive polarizations. When one surface is plated, one electrode is used as a positive polarization. When only one of the two electrodes is used as a positive polarization, the other is exposed to a poor potential region, and in some cases, it comes to be a negative polarization.
When used as an ordinary anode, the above electrode has sufficient durability. However, when the electrode is exposed to a poor potential region, the problem is that a catalyst is exhausted to a greater extent so that the durability of the electrode extremely decreases. The reason therefor is mainly as follows. The electrode is brought into a reduced state and the surface of the electrically conductive substrate is therefore embrittled due to hydrogen so that the coherence to a catalyst is removed, and iridium oxide having a high catalytic performance comes to be completely reduced.
As described above, practically, the electrode is not only required to have a durability as a positive polarization, but also required to have sufficient durability even when it is placed in a poor potential region. Various studies are therefore being made on methods of improving corrosion resistance in a reduced state, in which, in sulfuric acid electrolysis, platinum poor in corrosion resistance is added to decrease a hydrogen overpotential.
JP-A-5-230682 discloses an electrolyzing electrode comprising an electrically conductive substrate, an intermediate layer which is composed of a platinum layer containing platinum as a main component and an oxide layer containing oxides of valve metals (titanium, tantalum, niobium, zirconium and tin) as main components and an electrode activation material layer coated on the intermediate layer. This electrode has durability for a negative polarization. However, when it is used as an anode for electrolysis in an acidic solution of a sulfuric acid, an electrolytic solution infiltrates into the interior of the electrode to reach the platinum layer, and the platinum is exhausted, so that it is insufficient in durability.
Further, Japanese Patent Publication No. 2505563 discloses an electrolyzing electrode comprising an electrode substrate formed of titanium or a titanium alloy, an intermediate layer which is composed of platinum dispersed and coated on the electrode substrate at a coverage ratio of 10 to 80% and other metal oxides (0 to 20 mol % of iridium oxide, manganese oxide, cobalt oxide, tin oxide and antimony oxide and 80 to 100 mol % of niobium oxide, tantalum oxide and zirconium oxide) filling in spaces thereof, and an outer layer which is composed of 5 to 94 mol % of iridium oxide, 1 to 30 mol % of platinum and 5 to 94 mol % of oxide of valve metal and formed on the intermediate layer. This electrode has durability for a negative polarization. However, when it is used as an anode for electrolysis in an acid solution of a sulfuric acid, it is insufficient in durability since platinum being contained in both the intermediate layer and the outer layer is exhausted.
JP-A-5-255881 discloses an electrode for the generation of oxygen, which comprises an electrically conductive substrate, a platinum metal/tantalum oxide undercoating layer which contains, as metals, 1 to 20 mol % of platinum and 80 to 99 mol % of tantalum and is formed on the electrically conductive substrate, an iridium oxide/tantalum oxide intermediate layer which contains, as metals, 80 to 99.9 mol % of iridium and 20 to 0.1 mol % of tantalum and is formed on the undercoating layer, and an iridium oxide/tantalum oxide overcoating layer which contains, as metals, 40 to 79.9 mol % of iridium and 60 to 20.1 mol % of tantalum and is formed on the intermediate layer.
JP-A-8-225977 discloses an electrolyzing electrode comprising a titanium substrate, an alloy layer which is composed of titanium, platinum and tantalum and is formed on the titanium substrate, an intermediate layer which is composed of 5 to 30 mol % of iridium oxide and 70 to 95 mol % of tantalum oxide and is formed on the ally layer surface and an outer layer which is composed of 60 to 98 mol % of iridium oxide and 2 to 40 mol % of tantalum oxide and is formed on the intermediate layer.
Japanese Patent Publication No. 2505560 discloses an electrolyzing electrode comprising an electrode substrate formed of titanium or a titanium alloy, an intermediate layer which is composed of platinum dispersed and coated on the electrode substrate at a coverage ratio of 10 to 80% and other metal oxides (0 to 20 mol % of iridium oxide, manganese oxide, cobalt oxide, tin oxide and antimony oxide and 80 to 100 mol % of niobium oxide, tantalum oxide and zirconium oxide) and is formed on the electrode substrate, and an outer layer which is composed of 5 to 95 mol % of iridium oxide and 5 to 95 mol % of oxides of valve metals (niobium, tantalum and zirconium) and is formed on the intermediate layer.
The above electrodes have durability for a negative polarization to some extent. However, when the thickness of a catalyst layer (intermediate layer) is increased for attaining a longer lifetime of the electrode, the effect of platinum incorporated into the undercoating layer decreases, and the durability for a negative polarization decreases.
As described above, there has been obtained no electrolyzing electrode which has a long lifetime in ordinary electrolysis and has sufficient durability when placed in a poor potential region.